OBD-II TrckrX: data logging in a BMW E36 M3

[Bruce Land] sent in this cool final project for ECE 4760 at Cornell University. Dubbed TrckrX, it is an OBD-II tracking and data logging system built into a BMW E36 M3. The car in question is being used in some auotocross competitions. The driver wanted instant access to some data as well as a log of everything for later analysis. The unit gives a real time display of vehicle speed, coolant temp, and RPM. G-force and timestamps are stored on the SD card.

We think this is a very cool idea, and could be quite useful in some instances. The real time display of speed and RPM seem a bit peculiar as the car’s speedometer and tachometer are more appropriately placed for real time information. However, we completely understand that this was a class project and this person may not have wanted to replace their dash cluster with a new readout.

You dont have to have an OBDII car, this can also be done with earlier cars as well. 3000/Stealth group has had similar for a while. I have a palm pilot for my car that logs, displays and allows me to review data at any time using MMCD. Here is a similar variation of this type of tech on the 3/S platform.

Tip for you nerdy techie car guys. IT looks better if you mount the LCD flush and then put a clear piece of plexi over it. if you want to be even more “uber cool” put a bit of window tint on the back side to tint it (paint the non display back black) and you can with only 20 minutes extra work make it almost look stock.

obd II is generally laggy for rpm etc, newer CAn based OBD II is better, but can still lag especially since a lot of ECU’s have priorities for servicing requests, they’ll queue or ignore requests if something else is taxing or taking a higher priority.

That’s intentional, and usually it gets worse the faster you go. My car is only off by about 3 MPH at 60, but is off by 6 at 114 (120 indicated). I noticed this with a GPS logger running on a track day….

Some cars are even worse than that, I’ve seen upwards of 10 MPH at 80 or so on some vehicles. Basically the manufacturers know people want to go fast, so they make you seem faster the faster you go, to try to keep you from being quite so dumb. At least that’s the best I could ever figure it.

What charliex said.
In Germany I know it for certain and a speedo is under no circumstances allowed to show too little speed. There’s also a percentage threshold for far it may be ahead, of course.

When you think about the accuracy of speedos, though, keep in mind cars usually have several tire sizes they can use and they usually differ very slightly in circumference. Also, as the tires get used they also lose a bit of their circumference, so having an exact speedo would need to know the exact circumference of your tires.
Another effect is the dynamic change in circumference, as the centrifugal also very slightly changes circumference if you go faster. If you go really fast, the fact that normal civil cars have aerodynamics that tend to lift them from the street also starts playing a role.
Oh right, wheel slip also exists!

So the bottom line is that you can only trust the speedo as being measure of rotations per minute of your wheels, which is loosely related to how fast you go. Unless you cannot be sure that the manufacturer didn’t try to compensate for that, in which case you can’t trust the speedo at all when it comes to precision.

Hmm since I live in Europe and I can’t find microOBD 200, can anyone give me a tip where to find information how to replace it? (Some OBDII specifications, what protocols are used in what cars, how to ask ECU and how to receive and other things like this)

You can have this hack under 30USD, bluetooth OBD-adapter and Torque -software for Android. You can even have video recorded of you drive with user selectable dials overlayed. Just add torquerecorder plugin. Search youtube.

Do you have a BMW equipped with an OBC (the on board computer you are refering to) or you just assume that you can get instantaneous reading on it just because it has buttons saying speed and consum? Last time I checked the OBC doesnt give an instantaneous reading of either of the 3 – MPH, RPM or Temp of the Coolant. It can give you an average reading over time. Yes you do have a dashboard for the first two but for the coolant you just have a needle which is no very close to being useful. As soon as it moves from the center position you better pull over or your car is toast.